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Gardening Answers > Fall >

Dividing Perennials in the Fall

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General Dividing Tips for Perennials

Water really well, 1-2 days before dividing. Cut foliage back to 6” or half the plant to make it easier to cut.

Reasons Why We Divide Perennials

  1. Flower production is reduced
  2. Smaller flower heads
  3. Center of plant dies out
  4. Plant loses vigor
  5. Plant flops and now needs staking
  6. Plant has outgrown its bounds

3 Goals when Dividing Perennials

  1. Rejuvenating the plant so it can continue to perform the way it was intended
  2. Control the size of the plant
  3. Increase the number of plants

Plants to divide in late summer to fall:

Dividing Daylilies

Fall is a good time to thin and transplant crowded clumps. Separate and cut foliage back by 50%. Replant.

Be sure to water your plants extra in the days before you divide, they will need the strength to be transplanted successfully.

Dividing Hosta

After flowers have faded, dig up your hosta and wash away the soil from the roots. You’ll see lavender eyes that will become next year’s plant. Take a sharp knife and cut to one side of those eyes. Replant.

Planting & Transplanting Iris

The Greek goddess Iris walked a rainbow pathway through the sky and the flower named for her has a rainbow of flower color. This radiant flower was regarded as the symbol of light and is emblematic of promise, light and hope, pride and bravery.

The best time to plant and transplant Rhizomatous Iris is late July through September. Rhizomes are horizontally growing underground stems that are used as food storage for the plants. Rhizomatous Irises include the common bearded Iris as well as the beardless Siberian and Japanese Iris.

Iris love the heat and the drier wether of summer, and summer dividing will reduce the incidence of bacterial soft rot. Most Iris need to be divided every three to five years. If your plants are not producing many flowers, it’s time to DIVIDE and CONQUER!

Steps for dividing iris:

  • Cut back the foliage to about one-third its height.
  • Lift the entire clump with a spade.
  • Use a sharp knife to separate the rhizomes. Dip the knife in a ten percent bleach water mixture after each cut.
  • While dividing, make sure to inspect the rhizomes for soft rot and iris borer. Iris borer is the worst insect problem that can affect Iris. An Iris borer adult is a brownish moth and she lays her eggs in fall on iris leaves, which over winter there and hatch into caterpillars during April and May. The caterpillars first bore into the Iris leaves and by the end of July, the caterpillars move into the rhizome to eat and mature. In early August, the caterpillars move from the soil to pupate into a moth.
  • When dividing, the Iris borer will be a mature pink caterpillar inside the rhizome and it will be mushy to the touch. Bacterial soft rot often accompanies iris borer damage. If there is evidence of Iris borer, spray with Cygon the following spring.
  • To avoid any decay on the new breaks or cuts, dust the rhizomes with a powdered fungicide such as sulfur or Bulb Dust.
  • The new transplants should have a firm rhizome with roots and a fan of leaves.
  • Remove and discard the old rhizomes and replant the younger smaller rhizomes that grow off the older stems.
  • Replant in a sunny, well-drained garden spot. Dig the hole about 5 inches deep and build a small mound in the middle to place the rhizome.
  • Place the rhizome on the mound allowing the roots to fall down either side and cover the roots so that the rhizome is ever so slightly exposed. Do not plant rhizomes too deep or it may rot or not flower.
  • As a rule of thumb, Iris are planted 18-24” apart in groups of three to seven sections of one variety.
  • Typically the rhizomes are planted so the leaf fans face in one direction.
  • Fall sanitation is very important in the control of Iris borer. After the first hard frost, remove and destroy the old iris foliage and plant debris to remove any eggs.
  • Keep well watered, but not too wet.
  • Cover after the ground has frozen if you are dividing.
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